Thermal cracking of paraffins to aromatics



3,271,298 THERMAL CRACKING F PARAFFINS T0 AROMATICS Clarence L. Dulaney and Marvin Lee Owens, Jr., Texas City, Tex., assignors to Monsanto Company, a corporation of Delaware N0 Drawing. Filed Feb. 28, 1963, Ser. No. 261,878

10 Claims. (Cl. 208-106) The present invention relates to a process for the thermal cracking of hydrocarbons. Further, the present invention relates to a process for the production of aromatic hydrocarbons. More particularly, the present invention relates to a process for the thermal cracking of paraffinic hydrocarbons to obtain a cracked product of increased aromatic hydrocarbon content.

Because of the relatively low economic value of paraffin hydrocarbons, there is a continuous search for methods whereby these hydrocarbons may be converted into more valuable materials. Among the more valuable materials which may be produced from parafiins are olefin and aromatic hydrocarbons. Thermal cracking of paraflin hydrocarbons is one of the well known and widely used processes whereby the paraffins are converted to more valuable olefinic hydrocarbons. In addition to the olefinic hydrocarbons, a very small amount of aromatic hydrocarbons are usually produced by the thermal cracking of paraffins. However, the amount of aromatic hydrocarbons produced is generally so small that the value of the cracked products is not enhanced despite the higher value of the aromatic hydrocarbons. Because of the high value of the aromatics, the greater the amount of such hydrocarbons which can be produced from the thermal cracking of parafiins the greater the value of the cracked prodnot.

It is an object of the present invention to provide a new and improved process for the thermal cracking of paraflin hydrocarbons. Another object of the present invention is to provide a process for the thermal cracking of paraflin hydrocarbons whereby the quantity of aromatic hydrocarbons produced is significantly increased. Still another object of the present invention is to provide a process for the production of aromatic hydrocarbons from paraifin hydrocarbons. Additional objects will become apparent from the following description of the invention herein disclosed.

In fufillment of these and other objects, it has been found that there is a substantial increase in the yield of aromatic hydrocarbons from the thermal cracking of paraffin hydrocarbons when a minor amount of a cyclic hydrocarbon, selected from the group consisting of hydrocarbon substituted aromatic hydrocarbons partially saturated cyclic hydrocarbons and saturated cyclic hydrocarbons, is added to the thermal cracking zone concurrently with a paraflin hydrocarbon feed. The thermal cracking may be carried out under any conditions of temperature and pressure conventional to such processes. Most often the temperature will be within the range of 400 to 900 C. with temperatures of from 500 to 800 C. being preferred. The pressure usually is within the range of from atmospheric to 1000 p.s.i.g., but preferably within the rang of to 300 p.s.i.g. Ordinarily, the thermal cracking of parafiin hydrocarbon is carried out in the presence of an inert diluent, preferably steam. When an inert diluent is used it generally is used in an amount of 0.1 to 2.0 parts by weight of diluent per part by weight of hydrocarbon feed. Preferably, a diluent to hydrocarbon feed weight ratio of 0.2: l to 0.8 :1 is used in the present invention.

The cyclic hydrocarbons useful in promoting the formation of aromatic hydrocarbons in accordance with the present invention includes hydrocarbon substituted aromatic hydrocarbons, partially saturated cyclic hydrocar- States Patent bons and saturated cyclic hydrocarbons. The hydrocarbon substituted aromatics include both monoand polynuclear aromatics. Substituents to the aromatic nucleus may be aryl, alicyclic or acyclic and may be saturated or unsaturated. The primary limitation on the substituent hydrocarbons is that there be at least one hydrogen atom attached to the substituent carbon adjacent the aromatic nucleus. There may be any number of substituents to the aromatic nucleus such as in dialkylbenzenes, trialkylbenzenes and the like. Several non-limiting examples of hydrocarbon substituted aromatic hydrocarbons are toluene,

cumene,

styrene,

n-butylbenzene,

mesitylenes, l-methylnaphthalene, 1,2-dimethylnaphthalene, xylenes,

triethylbenzenes, 1,3-dimethylphenanthrene, 1,2-di-n-propylbenzene, 1-phenyl-4-methylpentane, cyclopentylphenylmethane, 1-cyclopentyl-Z-phenylethane, diphenylmethane, 1,1-diphenylethane, 1-phenyI-Z-methylpropene-Z, l-methyl-S-(propen-2-yl)benzene and the like.

A preferred group of substituted aromatic hydrocarbons are the monoand di-alkyl substituted benzenes and naphthalenes in which the alkyl substituents have no greater than 10 carbon atoms. These preferred hydrocarbon substituted aromatic hydrocarbons are illustrated by the following non-limiting examples: l-methylnaphthalene, toluene, cumene, Xylenes, isopropylbenzene, n-butylbenzene, dimethylnaphthalenes, dibutylbenzenes, methylethylbenzenes, dipropylbenzenes and the like. The partially satua rated cyclic hydrocarbons, for the purposes of the present invention, are the cyclic hydrocarbons more saturated than aromatics, but yet not completely saturated and includes both mono-nuclear and poly-nuclear compounds. The mono-nuclear cyclic hydrocarbons include the cycloolefins and cyclodiolefins. Poly-nuclear cyclic hydrocarbons include the dicycloolefins and dicyclodiolefins, and the fused ring aromatics in which at least one ring is partially or completely saturated, i.e., tetralin and the like. The partially saturated cyclic hydrocarbons are illustrated by the following non-limiting examples:

The preferred partially saturated cyclic hydrocarbons are the cyclodiolefins, cycloolefins and partially saturated dinuclear fused ring aromatics. This preferred group of compounds include the following non-limiting examples: cyclohexene, cyclopentene, methylcyclopentene, methylcyclohexene, cyclopentadiene, methylcyclohexadiene, dimethylcyclohexene, Tetralin, indane, and the like. Completely saturated cyclic hydrocarbons include the cycloparaffins, dicycloparaffins and the completely saturated fused ring cyclic hydrocarbons. Several non-limiting examples of such hydrocarbons are cyclopentane, cyclohexane, cycloheptane, methylcyclopentane, methylcyclohexane, decahydronaphthalene, methylbicyclodecane, ethylbicyclodecane, and the like. The preferred hydrocarbons within this group are the cycloparafiins of 5 and 6 carbon atoms in the ring and having either no substituents or having aliphatic hydrocarbon substituents of 1 to 6 carbon atoms and the saturated di-nuclear fused ring cyclic hydrocarbons of 8 to 12 carbon atoms such as decahydronaphthalene, either unsubstituted or having alkyl substituents of 1 to 6 carbon atoms.

The amount of cyclic hydrocarbon useful in the present, in most instances, is within the range of approximately 0.1 to mol percent of the paraffin hydrocarbons in the feed. However, it is preferred that the amount of cyclic hydrocarbon be within the range of from approximately 0.5 to 5 mol percent of the paraflin hydrocarbons in the feed. In many instances, the feedstocks to the present process may contain as an impurity small amounts of hydrocarbon substituted aromatics, partially saturated cyclic-hydrocarbons or completely saturated cyclic hydrocarbons. 'If such is present in the feed initially, the amount of such material added to the thermal reaction zone is reduced proportionately.

The feedstocks which may be processed in accordance with the present invention are paraflinic fractions containing 20 to 100% by weight of parafiin hydrocarbons. By paraffin hydrocarbons is meant the non-cyclic saturated hydrocarbons. Preferably the feedstock will be a paraflin fraction of 90 to 100% by weight parafiin hydrocarbons. The paraflin hydrocarbons may be relatively low molecular weight liquids or high molecular weight waxy solids. Generally, the paraffinic hydrocarbons will have at least 6 car-bon atoms and may be straightchain or branched-chain. Several non-limiting examples of parafiinic hydrocarbons within the scope of the present invention are n-hexane, 2-methylpentane, 3- methylpentane, n-heptane, 2-methylhexane, 3-methylhexane, Z-ethylpentane, n-octane, Z-methylheptane, 2-ethylhexane, 3-methylheptane, 3-ethylhexane, n-nonane, 2,2- dimethylheptane, 2-methyl-4-ethylhexane, 3,3-diethylpentane, n-decane, and the like on up to and including nand iso-paraffins of 70 carbon atoms and higher. The preferred parafiin hydrocarbons are those containing 6 to 40 carbon atoms and are either straight-chain or branched-chain. In parafiin fractions containing less than 100% paraflin hydrocarbons, the impurities may include any hydrocarbon such as paraffin hydrocarbons of less than 6 carbon atoms, olefins, aromatics, naphthenes and the like, as well as diluents and inert materials.

In carrying out the practice of the present invention, spaces velocities of 0.5 to 10 parts by volume of feed per part by volume of internal reaction space generally is used. However, it is preferred that the space velocity be Within the range of 2 to 6 parts by volume of paraffinic feed per part by volume of internal reaction space.

In order to demonstrate the efficacy of the present invention, the following examples are presented.

Example I The paraffinic feedstock used in this demonstration was 100% paralfinic hydrocarbons having a molecular weight range of about 60 to 700. This parafiinic feed was passed through 35 feet of inch stainless steel tubing at a rate of 4.4 pounds of feed per hour. The

parafiinic feed was passed into the reaction tube concurrently with steam in a ratio of 0.4 pound of steam per pound of feed. Five mol percent of toluene was introduced concurrently with the paraffinic feed and steam. The inlet temperature of the 35 foot reaction tube was approximately 450 C. and the exit temperature 602 C. The pressure within the reaction tube was maintained at approximately 11.3 p.s.i.g. A conversion of 73.5% 'was obtained. A C -C fraction was obtained from the cracked oil product by distillation. This fraction represented 14.5% of the total product and was found to contain 7.4% by weight of aromatic hydrocarbons. The percent of aromatic hydrocarbons was calculated on a toluene-free basis.

Example 11 Example I was substantially repeated with the exception that 5 mol percent of cumene was added and the feed rate was 4.6 pounds per hour. Conversion was approximately 76.4%. A C -C fraction was obtained from the cracked oil product by distillation. This fraction represented 11.0% by weight of the total cracked oil product and was found to contain 4.2% by wieght of aromatic hydrocarbons. The percent of aromatic hydrocarbons was calculated on a cumene-free basis.

Example 111 Example I was substantially repeated with the exception that 5 mol percent of Decalin was added and the feed rate was 4.7 pounds per hour. Conversion was approximately 77.5%. A C C fraction was obtained from the cracked oil product by distillation. This fraction represented 12.8% by weight of the total cracked oil product and was found to contain 4.37% by weight of aromatic hydrocarbons. The percent of aromatic hydrocarbons was calculated on a Decalin-free basis.

Example IV Example I was substantially repeated with the exception that no cyclic hydrocarbon was added. Conversion was approximately 73%. The cracked oil product was fractionated to obtain a C -C fraction. This fraction represented 9.2% by weight of the total cracked oil product and was found to contain 1.5% by weight of aromatic hydrocarbons.

From the above examples, it is quite apparent that the present invention produces substantially increased yields of aromatic hydrocarbons. Further, it should be noted that in both instances in which cyclic hydrocarbons were added that significantly improved conversions of paraffinic hydrocarbons to cracked products were obtained.

The equipment which may be used in carrying out the present invention is not critical. Any conventional thermal cracking equipment may be used. It is only necessary that the equipment 'be such as to withstand the pressures and temperatures of the reactions and that the equipment follow good engineering principles.

What is claimed is:

1. A process for increasing the production of aromatic hydrocarbons from the thermal cracking of paraffin hydrocarbons which comprises adding 0.1 to 10 mol percent of a hydrocarbon selected from the group consisting of hydrocarbon substituted aromatic hydrocarbons, partially saturated cyclic hydrocarbons and saturated cyclic hydrocarbons to a noncatalytic thermal cracking zone concurrently with a paraffin hydrocarbon feed containing less than 1% of non-parafiin hydrocarbons, said cracking zone being maintained at a temperature of 400-900 C. and a pressure of atmospheric to 1000 p.s.1.g.

2. The process of claim 1 wherein the thermal cracking is carried out in the presence of an inert diluent.

3. The process of claim 1 wherein the parafiin hydrocarbons in the feed contain 6 to 40 carbon atoms.

4. The process of claim 2 wherein the inert diluent is present in an amount of from 0.1 to 2.0 parts by weight of diluent per part by weight of hydrocarbon feed.

5. The process of claim 2 wherein the inert diluent is steam.

6. The process of claim 1 wherein the thermal reaction zone is maintained Within the temperature range of from 500 to 800 C. and the pressure is maintained within the range of from 5 to 300 p.s.i.g.

7. The process of claim 1 wherein the amount of cyclic hydrocarbon added is approximately 0.5 to 5 mol percent of the paraffin hydrocarbons in the feed.

8. The process of claim 1 wherein the cyclic hydrocarbon is a hydrocarbon substituted aromatic hydrocarbon selected from the group consisting of monoalkylbenzenes, monoalkylnaphthalenes, dialkylbenz/enes, and dialkylnaphthalenes and is one having no greater than 10 carbon atoms in the hydrocarbon substituents.

9. The process of claim 1 wherein the cyclic hydrocarbon is a partially saturated cyclic hydrocarbon selected from the group consisting of cycloolefins, cyclodiolefins and partially saturated di-nuclear fused ring aromatics.

References Cited by the Examiner UNITED STATES PATENTS 6/ 1945 Dorsett et al. 208132 9/1958 Smith et al. 208-132 DELBERT E. GANTZ, Primary Examiner.

ALPHONSO D. SULLIVAN, Examiner.

H. LEVINE, Assistant Examiner. 

1. A PROCESS FOR INCREASING THE PRODUCTION OF AROMATIC HYDROCARBONS FROM THE THERMAL CRACKING OF PARAFFIN HYDROCARBONS WHICH COMPRISES ADDING 0.1 TO 10 MOL PERCENT OF A HYDROCARBON SELECTED FROM THE GROUP CONSISTING OF HYDROCARBON SUBSTITUTED AROMATIC HYDROCARBONS, PARTIALLY SATURATED CYCLIC HYDROCARBONS AND SATURATED CYCLIC HYDROCARBONS TO A NON-CATALYTIC THERMAL CRACKING ZONE CONCURRENTLY WITH A PARAFFIN HYDROCARBON FEED CONTAINING LESS THAN 1% OF NON-PARAFFIN HYDROCARBONS, SAID CRACKING ZONE BEING MAINTAINED AT A TEMPERATURE OF 400-900*C. AND A PRESSURE OF ATMOSPHERIC TO 1000 P.S.I.G. 